The invention relates to the field of nuclear magnetic resonance (NMR), in particular to NMR imaging methods and systems. Even more particularly, the invention relates to the enhancement of magnetic resonance (MR) image contrast.
The publications and other materials, including patents, used herein to illustrate the invention and, in particular, to provide additional details respecting the practice are incorporated herein by reference. For convenience, the publications are referenced in the following text by author and date and are listed in the appended bibliography.
A physician generally checks for the presence of tumors and other tissue abnormalities by palpating a patient and feeling differences in the compliance or “stiffness” of soft tissues, such as muscles, brain, liver and adipose tissue, thereby detecting potential irregularities. Palpation is in fact the most common means for detecting tumors of the prostate gland and the breast. However, unfortunately, deeper lying structures are not accessible for evaluation via palpation.
Magnetic resonance elastography (MRE) offers a non-invasive diagnostic method that extends the physician's ability to assess the mechanical properties of soft tissues throughout a patient's body (R. Muthupillai et al., Science 1995, and R. Muthupillai et al., Magn Reson Med, 1996). Many pathologies of these tissues cause a change in their mechanical properties and a number of studies have demonstrated the potential use of quantitative measurements of tissue elasticity to identify the affected region, such as in the prostate (M. A. Dresner et al., Proc. ISMRM, 1999 and R. Sinkus et al., Proc. ISMRM, 2003), head (J. P. Felmlee et al., Proc. ISMRM, 1997; S. A. Kruse et al., Proc. ISMRM, 1999; J. Rydberg et al., Proc. ISMRM, 2001; and J. Braun et al., Proc. ISMRM, 2002), skeletal muscle (M. A. Dresner et al., J Magn Reson Imaging, 2001; I. Sack et al., Magn Reson Imaging, 2002; and K. Uffmann et al., Proc. ISMRM, 2002), and breast (A. J. Lawrence et al., Proc. ISMRM, 2002; R. Sinkus et al., Phys Med Biol, 2000; and E. E. van Houten et al., J Magn Reson Imaging, 2000).
U.S. Pat. No. 5,757,185 to Hennig describes a method based on a principle similar to MRE, namely, on the motion of charged particles or time-dependent magnetic fields generated by moving particles which are measured with phase sensitive imaging methods to assess the properties of soft tissue throughout a patient's body. Two radio-frequency (RF) pulses are applied to a sample located in a homogeneous external magnetic field and a spatially variable magnetic gradient field (GF) is applied in the time interval between the first RF pulse exciting the NMR signal and the read-out of the corresponding signal. The magnetic gradient field (GF) is chosen so as to cause a velocity-dependent change in the phase and/or amplitude of the signals from spins moved in the direction of the magnetic field gradient by application of external electric fields to the sample. The techniques are known as Magnetic resonance current density imaging (MRCD, G. C. Scott et al., 1989) or Magnetic resonance electrical impedance tomography (MREIT, E. J. Woo et al. 1994). The information gathered by these methods about electrical conductivity of a biological tissue is useful for many purposes, such as for locating the source of electrocardiogram (ECG) and encephalogram (EEG) signals, modelling tissues to investigate action potential propagations, estimating therapeutic current distribution during electrical stimulation, and monitoring physiological functions.
During MRE measurements, a stress which varies periodically in magnitude is used to impart mechanical motion to spins to produce shear waves which have wavelengths that are determined by/correlate to the mechanical properties of the sample.
Similarly, during MRCDI/MREIT measurements, a time-dependent electric field which varies periodically in magnitude is used to induce motion to charged particles. This induces time-dependent magnetic fields or motion-related spin dephasing in the sample being tested that relate to the sample's electrical conductivity.
As disclosed in U.S. reissue Pat. No. RE 32,701 to Moran, NMR can be used to detect and image the movement of spins. NMR signals can be sensitized to detect moving spins by applying a bipolar magnetic field gradient (GF) at the appropriate time in each NMR measurement sequence. The phase of the resulting NMR signal measures the velocity of spins along the direction of the motion sensitizing field gradient. Generally, the method is based on the differing development of the signal phase of stationary and moving particles in a bipolar magnetic field gradient along the direction of motion (see L. E. Crooks, N. M. Hylton in Vascular Diagnostics, eds. P. Lanzer and J. Rosch, Springer Verlag, Heidelberg, 1994, p. 366). The signal phase of stationary nuclei return to their initial value after application of a bipolar magnetic field gradient, whereas the signal phase of moving nuclei undergoes a dephasing proportional to the velocity of motion as well as to the strength of the magnetic field gradient, thus resulting in a spatial phase dispersion. Small cyclic displacements, e.g., on the micron to submicron level, can be detected by synchronizing a motion sensitizing gradient (MSG), which constitutes a combination of bipolar gradients, with an oscillating mechanical excitation of the tissue of interest (see U.S. Pat. Nos. 5,592,085 and 5,825,186). Images of low frequency transverse acoustic strain waves in tissue are generated using gradient-echo or spin-echo MR phase contrast techniques (R. Muthupillai et al., Magn Reson Med, 1996, and R. Sinkus, Phys Med Biol, 2000). These techniques are applied in combination with MSG for several cycles in synchronization with the applied stress to allow for significant phase accumulation. The cyclic MSG waveforms can be applied along any desired axis with variable frequency and number of gradient cycles and normally two acquisitions are made in an interleaved fashion to reduce systematic errors. Consequently, standard MRE experiments, as, e.g., disclosed in U.S. Pat. Nos. 5,592,085 and 5,825,186, suffer from long acquisition times due to the time needed for magnetization preparation for motion sensitizing, followed by a single readout of NMR signals. Furthermore, there is some technical demand for synchronization of MSG gradients with the mechanical excitation. Similar considerations apply to the generation of MRCDI/MREIT images.
Contrast enhancement in nuclear magnetic resonance (NMR) imaging can extend its diagnostic potential.
Thus, there is a need for a system and method that provides an improved contrast mechanism for NMR images which reveals the mechanical and/or electrical properties of a sample.
There is also a need for a system and method that provides a very fast modality for generating NMR images revealing mechanical and electrical properties of the sample/object, such as stiffness or electrical impedance with high sensitivity.
There is furthermore a need for a system and method that adds contrast and enhances the sensitivity by subtraction and/or summation of steady-state images.
There is also a need to reduce the technical demand posed by synchronization of MSG and mechanical motion of spins or charged particles.